94 



pearance, being gradually eaten away and frayed out from the 

 surface by something present in the cells, and this has been 

 especially seen in observations made on seeds. The starch 

 grains do not dissolve entirely at once, but gradually dissolve 

 from the outside, in somewhat the same manner as sugar does 

 in hot water, its solution being due to the action of a diastatic 

 ferment. 



We -see, then, that the direction of the slow movement of 

 fluid is determined by local circumstances, and the reason it 

 does descend to the root and not ascend to the inflorescence is 

 simply in relation to the demand for sugar there. Wherever 

 the simple unaltered parenchyma tissue remains in the plant, 

 with unaltered cell-walls through which this diffusion may 

 prevail, there we may have this sugar solution passing ; conse- 

 quently we find it in certain strands of these cells which run at 

 intervals from the pith to the inner layer of the bark (bast 

 layer), and which are known as the " medullary rays ;" its 

 usual course is through the growing cells of the cambium zone. 



The slow movement of fluid is, then, of an essentially dif- 

 ferent nature from the rapid movement due to transpiration 

 and the other causes which we noticed. I do not think that this 

 movement of the sugar solution is in any way a return current 

 downwards of the remainder of the fluid which, with mineral 

 salts in solution, being deprived of its nutrient substances as it 

 goes on its way, at length finds its way to the leaves, and there 

 deposits in the leaf-cells the salts which it contains, which are of 

 no further use in the plant's nutrition, the water passing off in 

 the form of watery vapour through the stomata ; but it is a new 

 current, set up, in obedience to the laws of diffusion, towards 

 the points where there is a demand, and traversing the par- 

 enchymatous tissues where such laws hold complete sway. 



In addition to the sugar solution we. have certain other bodies 

 — viz., the crystalline nitrogenous bodies, such as asparagin, 

 leucin, and tyrosin, which are formed as the products of a long 

 series of transformation changes on the proteids. These are 

 readily diffusible, and can pass with ease, in solution in water, 

 from cell to cell through the cell-walls of the parenchymatous 



